Referring to FIGS. 3 and 4, a less preferred embodiment of an autofocus bar code reader, generally designated by the reference numeral 30, is explained. A conveyer 31 transports an article 32 having attached thereto a bar code label 33. A distance measuring section 9 measures distance 34 to the bar code label 33 and generates a distance information pulse signal 17 indicative of the measured distance. The distance information pulse signal 17 is fed to a is focal position controlling section 8. A laser beam generating section 35 generates circular shape laser beam 16. In response to the distance information pulse signal 17, the focal position controlling section 8 conducts a focal position control so that the circular shape laser beam 16 will have its focal position on the bar code label 33. The circular shape laser beam that has been subjected to the focal position control is used as a scan laser beam 18 upon lighting the bar code label 33. Reflected light 19 by the bar code label 33 is intercepted and then decoded to obtain information on the bar code label 33. The distance measuring section 9 determines the distance 34 to the bar code label 33 by calculation after inputting result of detection of height of the article 32 by a multi-optical-axis sensor. The multi-optical-axis sensor includes a light emitter 36 and a light interceptor 37 between which a number of parallel optical beams extend. The light interceptor 37 detects the optical beams, in number, which are interrupted by the article 32 and provides the output indicative of the detected result to the distance measuring section 9.
In the bar code reader of the above kind, the diameter of the scan laser beam 18 is determined in accordance with the width of the narrowest bar of the bar code label 33. If the laser beam diameter is too large as compared to the width of the narrowest bar, a reduction in resolution occurs, making it impossible to detect the width of each bar with good accuracy, causing a drop in read performance. If the laser beam diameter is too small as compared to the width of the the narrowest bar, there is the tendency to detect fine defects and/or small dirty spots on a bar code label, causing a drop in read performance. It is therefore generally known to adjust the diameter of the scan laser beam upon being intercepted by a bar code label on its face as large as the width of the narrowest bar of the bar code label. Read operation under this condition may be called read operation with the optimized beam diameter. As shown in FIG. 2, the diameter 40 of a laser beam 41 is the smallest at its focal position 42. Therefore, it is the common practice to adjust the beam diameter equal to the above-mentioned optimized beam diameter at the focal position 42 that lies on the face of a bar code label to be read.
The preceding description explains why the distance to a bar code label is measured in laying the focus of laser beam on the face of the bar code label upon reading the bar code label in the case where the distance is subject to variations.
In order to maintain high read performance of a bar code reader, it is demanded to eliminate or at least reduce deviation from the optimized beam diameter.
Referring to FIG. 4, the laser beam generating section 35 is further described to clarify what causes the deviation from the optimized beam diameter. The laser beam generating section 35 includes a semiconductor laser diode 1, a collimating lens 2, a cylindrical lens a 3, and a cylindrical lens b 4, and generates a shaped laser means in the form of a circular shape laser beam 16. It has been found that variation in the ambient temperature of the laser beam generating section 35 causes variation in the focal position of the circular shape laser beam 16, thus inducing occurrence of the above-mentioned deviation. As is readily seen from FIG. 2, any deviation from the focal position 42 causes the laser beam diameter 40 to increase, causing a drop in read performance.
This phenomenon is observed in a bar code reader that employs a semiconductor laser diode as a source of laser beam when the ambient temperature is subjected to variation. As shown in FIG. 4, the semiconductor laser diode 1 generates an elliptical shape divergent laser beam 14 having different angles of divergence 50 in different directions. The maximum value of the angles of divergence 50 is around 60 degrees. The collimating lens 2 converts the elliptical shape divergent laser beam 14 to a collimating laser beam 15.
Distance a between the semiconductor laser diode 1 and the collimating lens 2 may be increased by using, as the collimating lens 2, a lens with increased aperture. The use of such lens result in an increase in accommodation space for the collimating lens 2 and an increase in manufacturing cost. The semiconductor laser diode 1 generates the elliptical divergent laser beam at a low output level. Thus, it is necessary to collect the entire laser beam within the angles of divergent 50 to provide the shaped laser beam at a sufficiently high output. This is the reason why the lens with a large aperture is required to collect all of the beams emitted by the semiconductor laser diode 1. The collimating laser beam 15 still has an elliptical cross sectional profile. Thus, two cylindrical lenses 3 and 4 are provided to convert the collimating laser beam 15 to the circular shape laser beam 16 with a circular cross sectional profile. The two cylindrical lenses 3 and 4 have a short focal distance. Two cylindrical lenses with a long focal distance may be used. In this case, an increase in accommodation space for the cylindrical lenses 3 and 4 and an increase in manufacturing cost result.
The semiconductor laser diode 1 and the lenses 2, 3 and 4 are mounted to and assembled with a casing made of aluminum. An increase in the ambient temperature causes thermal expansion of the casing. This expansion causes a change in distance between the semiconductor laser diode 1 and the collimating lens 2, a change in distance between the collimating lens 2 and the cylindrical lens a 3, and a change in distance between the cylindrical lens a 3 and the cylindrical lens b 4. In the case where the lenses with a short focal distance are used, these changes cause a substantial deviation of the focal distance of the circular shape laser beam 16 from the distance to the bar code label.
This phenomenon may be explained by the fact that, in a formula for a single lens with a focal distance f, (1/a)+(1/b)=1/f, a small change in the variable a causes a great change in the variable b. In FIG. 4, the collimating lens 2 collimates the elliptical shape divergent laser beam 14. This may be expressed by the formula after substituting the variable a with the distance a and the variable b with infinite. The collimating laser beam has its focal position spaced by infinite distance. Thus, the variable b in the formula is infinite. In this case, the term 1/b in the formula is zero to give the relation that a=f.
FIG. 5 shows the results of calculation of the variables f, a and b. As readily seen from FIG. 5, the shorter the focal distance f is, the higher is the rate of reduction in the variable b with respect to an increase in the variable a. In other words, with the same increase in the variable a, the rate at which collimating light converges increases as the focal distance f of a lens decreases. This means that the laser beam 15, which is to be collimated, tends to converge in response to an increase in the distance between the semiconductor laser diode 1 and the collimating lens 2. Further, the shorter the focal distance of the collimating lens 2 is, the higher is the rate at which the focal position of the collimating laser beam 15 changes in response to a change in the distance between the semiconductor laser diode 1 and the collimating lens 2. It is the practice to adjust the focal distance of the circular shape laser beam 16 to a designed distance based on the assumption that the laser beam 15 is a collimating light. Thus, if the laser beam 15 converges, the focal distance of the circular shape laser beam 16 varies.
The use of the semiconductor laser diode 1 as the source of is intended to accomplish minituarization and cost reduction of the bar code reader. Thus, it would be required to use lenses with a short focal distance in shaping the laser beam. As mentioned before, the use of the lenses with a short focal distance poses the problem that a small change in the distance a due to thermal expansion causes a substantial change in the focal position of the circular shape laser beam 16. This change in the focal position of the circular shape laser beam 16 causes a change in the foil position of the scan laser beam 18. Thus, the autofocus bar code reader shown in FIG. 3 has potential problem that, when used in the environment of varying temperatures, it would be difficult to keep the focal position of the scan laser beam 18 on the bar code label 33 with good accuracy.
In the preceding description, attention has been paid to a change in the distance between the semiconductor laser diode 1 and the collimating lens 2 as the cause of occurrence of substantial change in the focal position of the scan laser beam 18. Similarly, a change in distance between the collimating lens 2 and the cylindrical lens a 3, and a change in distance between the cylindrical lens a 3 and the cylindrical lens b 4 causes a change in the focal position of the circular shape laser beam 16. However, this change in the focal position of the circular shape laser beam 16 is small and negligible. The focal position controlling section employs lenses. But, these lenses have a long foal distance so that a change between the adjacent two lenses has little influence on a change in focal distance.
If, as the source of laser beam, a laser tube is used, the collimating lens and the cylindrical lenses are no longer required. In this case, there is no potential problem of the above-mentioned kind due to the thermal expansion.
An object of the present invention is to improve the bar code reader of the above kind such that the influence of temperature on performance of the device is eliminated to allow the use of a semiconductor laser diode as a source of laser beam.